Author of "Thus Agur breathed his warm desire" in A Collection of Psalms and Hymns for the Use of Universalist Societies and Families 16ed. In some hymnals, the editors noted that a hymn's author is unknown to them, and so this artificial "person" entry is used to reflect that fact. Obviously, the hymns attributed to "Author Unknown" "Unknown" or "Anonymous" could have been written by many people over a span of many centuries.
Anonymous